DOC: document how to run performance tests

[jorisvandenbossche]

To not forget what @jreback said here: #6193 (comment)

[lexual]

If we're talking about vbench, it definitely needs some docs improvements.

Struggling through getting this running locally at the moment.

[jorisvandenbossche]

yes, indeed (and with test_perf.sh). If you can try to document it a bit while trying to get it running, that would be fantastic!

[jreback]

I think the contributing docs are sufficient at this point